1. Field of the Invention
This invention relates in general to the field of implanted bone fixation devices and in particular to a hip screw for operative reduction and internal fixation of femoral head, neck or intertrochanteric fractures which allows relative sliding of the device and yet prevents rotation of the broken portion of the bone following surgery.
2. Description of the Prior Art
In one of my prior patents, specifically U.S. Pat. No. 4,432,358, Compression Hip Screw Apparatus, issued Feb. 21, 1984, I described the problems associated with hip screws which were designed for use with certain types of fractures of the upper femur such as the head, neck or intertrochanter fractures. In general, these problems involve prevention of the femoral head from rotating relative to the femur during the postoperative period and allowing further impaction of the broken bone portions during the postoperative period.
As explained in the referenced patent, it is highly desirable to "key" the lag screw to the barrel plate in order to prevent femoral head rotation relative to the femur during the postoperative period when the hip joint is being normally used such as when walking, going up stairs, and other like body motions. Any tendency for the head to rotate relative to the femur must be resisted by the union formed between the broken bone and the implanted compression hip screw. Without support from the compression hip screw, there is the distinct possibility of failure of the fragile union of the fracture during the postoperative period. The key-keyway feature of certain compression hip screws served to prevent such rotation. Since the femoral head is attached to the lag screw and the femur is attached to the barrel plate, keying the lag screw to the barrel plate in effect keys the head to the femur. Unfortunately, the key-keyway devices created other problems which make their use less desirable. Friction between the key-keyway prevents relative motion between the lag screw and the barrel plate and thereby tends to prevent additional impaction of the fusioned bone fragments following surgery. Postoperative impaction is highly desirable. Since the key-keyway requires fairly close mechanical tolerances between the fitting parts, the fitup between the lag screw and the barrel during surgery is quite difficult. Any slight misalignment resulting from the preliminary hole drilling operations causes improper alignment of the shaft of the barrel plate relative to the shaft of the femur as well as improper alignment between the axial centerline of the lag screw and the centerline of the barrel. Any such misalignment results in bending movement and stresses being induced in the lag screw and the barrel plate. Ultimately, however, these stresses and bending loads are transmitted to the femur and the unionized fracture, which is obviously undesirable, and may lead to failure of the unionized fracture.
A number of attempts to minimize the adverse effects of the key-keyway design have in the most part proved relatively unsuccessful. Shortening of the key tends to reduce binding caused by friction, but does not eliminate the tendency to bind. Extensions added to the distal end of the lag screw allows the operating surgeon to "find" the lag screw during placement of the barrel plate but the added parts increase the overall stackup of tolerances increasing the possibility of axial misalignment between the screw and barrel which tends to increase the resulting bending movements and loads which must be carried by the fusioned bone fragments. Thus, the key-keyway design of compression hip screw has not provided the answer to the problem of prevention of femoral head rotation. As a result, and obviously a compromise, there has been a recent tendency for surgeons to return to the original keyless design which, of course, does not address the head rotation problem but does allow for low friction sliding during the postoperative period.
My prior referenced patent, disclosed a new and different compression hip screw which does overcome the abovestated problems. A unique locking assembly is used to frictionally lock the lag screw, the compression nut and the barrel of the hip plate. While shown to be effective, there still exists the need to provide new and improved compression hip screws which are simple in terms of surgical procedures, are effective in terms of impaction during and after surgery, and are effective to maintain fusion of the broken bones or fragments during the postoperative period when the device is being used. The present hip compression screws disclosed and claimed herein accomplish these objectives.